1. Field of the Invention
Fiberscopes with particular reference to those formed of fused together optical fibers having leachable overcladdings.
2. Description of the Prior Art
In the manufacture of flexible optical fiber light and/or image conduits produced by leaching matrix glasses from between intermediate portions of the lengths of the fibers according to the techniques shown and described in U.S. Pat. Nos. 3,004,368 and 3,624,816, for example, it is a practice to retain the matrix glasses at opposite ends of the conduits. This forms permanent connections between the fibers and, in the case of image conduits which are called "fiberscopes", it keeps corresponding opposite ends of the fibers in prearranged substantially identically mosaically patterned relationships with each other.
In tightly packed bundles of fibers having clear light-conducting core and cladding components, light entering cores of the fibers at angles greater than the light acceptance angles of the particular fibers will tend to be transmitted into and through the fiber claddings and become scattered into other fibers of the bundle, eventually at least partially reaching the opposite end of the bundle as stray light. This, of course, does not contribute to reproduction of a mosaic light image being transmitted through fiber cores of the fiberscope but rather tends to wash out or reduce contrast between segments or elements of the mosaic image. The light acceptance angle or aperture angle i of an optical fiber is controlled by the indices of refraction n.sub.1 of the core of the fiber and n.sub.2 of a cladding joined to the core in accordance with the following equation: EQU Sin i = .sqroot.n.sub.1.sup.2 - n.sub.2.sup.2.
Attempts to reduce stray light in optical fiber devices and thereby improve image definition have included a use of metallic and other opaque or semiopaque coatings on individual fibers with the defeative effect of causing an absorption of excessive amounts of useful or image-forming light. Approaches to the problem of reducing stray light in optical fiber devices with less sacrifice of image-forming light have involved the inclusion of interstitial light-absorbing filaments. These filaments are separated from the light-conducting core components of the optical fibers by the lower refractive index fiber claddings as shown and described in U.S. Pat. No. 3,247,756, for example. The interstitial light-absorbing filaments must, however, be formed of a material (e.g. glass) which is both optically and thermally compatible with all other glasses in the system. In addition to this limitation, the use of individual absorbing filaments which require a separate manufacturing operation in the making, are difficult to handle in preparing the assembly of the optical fiber bundle configuration.
Another somewhat more effective but more complicated and expensive form of absorbing interstitial element is illustrated in U.S. Pat. No. 3,387,959. This involves the addition of an outer clear cladding around each absorbing filament or the incorporation of an unclad filament into each optical fiber structure in a manner such that the filament is surrounded by a sufficient thickness of fiber cladding material to prevent its interference with image-forming light being conducted through the core part of the fibers.
The foregoing and similar prior art approaches to minimizing stray light in optical fiber image-conducting devices are relatively complicated, and uneconomical in that they require compatibility both optically and thermally of the light-absorbing elements with other glasses of the system and strategic positioning of the absorbing elements in the systems for maximum effectiveness. All of this is impractical to implement, if not inapplicable to the manufacture of image-conducting fiber optical devices by the aforementioned highly desirable, less complex and less expensive leaching technique shown and described in U.S. Pat. Nos. 3,004,368 and 3,624,816.
Accordingly, it is a principle object of the present invention to provide simple, straightforward and inexpensive means and method for minimizing stray light and its adverse effects in leached and leachable fiber optical fiberscopes and like devices.